Hello Karen
I had similar problem a couple of weeks back after my first long run of 6 miles. I had regularly done 3 or 4 miles before that... anyway, my left knee was a bit puffy and felt like it was locking - although it didn't hurt and was just an odd feeling.
Took rest of week off and checked with doctor who told me it was my age !!!!! (34) and that road running would put strain on knee.
Anyway, it has been fine with no problems and I have done 6 miles twice since. I think I just went too far and my knee complained about it!
Interestingly, my doctor said there was no evidence to suggest cod liver oil helped with bones/joints - but me not believing him, I am taking it anyway.
Just rest it for a while and start gradually, and don't over do it. Les